ast: Add TopLevelDecl.
ast: Reorganize code.
types: Update doc comments.
ast: Add PrimaryExpr.
ast: Replace Operand with PrimaryExpr.
ast: Add FuncLit.
ast: Add OperandName.
ast: Add MethodExpr.
ast: Add ParenExpr.
ast: Add Conversion.
ast: Add CallExpr.
ast: Add SelectorExpr.
ast: Add IndexExpr.
ast: Add SliceExpr.
ast: Add TypeAssertion.
ast: Add Stmt and SimpleStmt.